Magha is the third lunar month of the old Indian calendar.
It usually falls somewhere toward the end of February or early March.

The origin story
of this festival tells of a spontaneous gathering, from many separate
parts of the countryside, of 1250 fully enlightened monks. They had
all quite independently decided to go and visit the Buddha and assembled
on the full moon day of Magha in the Bamboo Grove at Rajagaha where
the Buddha was currently living. All of these monks were direct disciples
of the Buddha - having been ordained by him. Because of this gathering
the full moon of Magha has come to be known as 'Sangha day' and is a
time when monastics will make an effort to gather together to share
their knowledge and experiences. In the West this is toward the end
of winter when many of the monasteries have just finished a long retreat
and such a gathering is a very joyous time. Many
would not have seen each for some time and with the arising of spring
and the end of a long retreat there is much to share.

It is said that
on the last year of his life, on the full moon of Magha, the Buddha
declared that he would pass away in three months time. Whether or not
the tradition of commemorating this gathering had been established at
this point is unknown but the Buddha might have used such an event to
make this announcement. Regardless of this possibility such an auspicious
declaration adds significance to this moon day.

The day is a
consideration of what it means to be part of sangha - this includes
the fourfold sangha; lay men and women, monks and nuns but because of
the origin of the event it tends not to be so significant for lay people.
For the ordained community who have come together there may be a series
of meetings to discuss various aspects of the teachings, periods of
group meditation, talks given by senior members of the community (both
resident and visiting) and a variety of other events - often quite spontaneous
- over a period of several days.